1. Field of the Invention
This invention relates generally to methods and apparatus for color image reproduction, and in particular, it relates to methods and apparatus for preferred hue selection to optimize color image reproduction.
2. Description of Related Art
Color image reproduction is more widely available in modern days, thanks to the advanced development in color printing technologies and equipment. However, it has always been a challenge for a color printing process to accurately and precisely reproduce colors from one device's (such as a color monitor) color space to another device's (such as a color printer) color space.
The term “color space” is often used to describe a range of colors. A color space is usually defined by selected primary colors. For example, to create a three-dimensional (3-D) representation of a color space defined by the colors of red, yellow and blue, the amount of red color may be assigned to the representation's X-axis, the amount of yellow to its Y-axis, and the amount of blue to its Z-axis. The resulting 3-D space provides a unique position for every possible color that can be created by combining those three colors.
Different types of color image devices may have different color spaces. For example, a color scanner may use an RGB color space defined or created by the red (R), green (G) and blue (B) colors, while a color printer may use a CMYK color space defined or created by the three primary colors of pigment cyan (C), magenta (M), yellow (Y), and black (K)).
The term “gamut” is often used to refer to the range of colors that a device, such as a color scanner or printer, can produce. It is often desired to use one type of color image devices having one color space to reproduce color images from another type of color image devices. For example, it is often desired to reproduce a color image generated by a scanner on to a print medium by a color printer. However, it is likely that many colors in the scanner gamut are outside the gamut of the printer. Colors outside the reproduction range of an output device are referred to as “out of gamut” colors which cannot be reproduced exactly. Instead, some “in gamut” color that is within the reproduction range of the printer must be substituted for each “out of gamut” color in order to reproduce the image. This substitution of colors is often performed by gamut mapping algorithms. Examples of gamut mapping algorithms can be found in U.S. Pat. No. 6,956,581 issued to Cui et al., U.S. Pat. No. 7,379,208 issued to Henley et al., and U.S. Pat. No. 7,397,588 issued to Sloan et al. The concepts of “hue” and “chroma” and related concepts of “hue angle” (or “hue slice”) are often used in connection with these exemplary gamut mapping algorithms. However, the concept of quantifying aspects of the hue angle (or hue slices) has not been used in these algorithms for evaluation of the output gamut and to determine which hue to use for the preferred color.
Some color mapping algorithms, such as the one disclosed in U.S. Pat. No. 6,594,388 issued to Gindele et al., perform certain modifications to the preferred hue color based on a predetermined set of parameters. Nonetheless such algorithms did not contemplate modifying the preferred hue color based on the size/shape of the output color gamut at specified preferred hues predetermined by the end user.
It has also been known that human memory of certain colors (known as “memory colors”, e.g., skin-tone, foliage, etc.) deviates from the actual colors. Memory colors often have different hues and enhanced colorfulness compared with the actual colors, and it has been shown that human eyes often prefer color reproductions that are closer to the memory colors than to the actual colors. Accordingly, for certain specific color rendering intents, such as perceptual rendering or photographic rendering, it is important to accurately and precisely reproduce certain memory colors (e.g., grass green, sky blue, etc.), and it is also necessary to facilitate a highly dynamic lightness range for color reproduction. The purpose of such a range would be to avoid gradation and detail losses as well as tone jumps for specific regions of the gamut.
Therefore, it is desirable to provide a method for selecting a preferred hue that would be able to give the highest amount of lightness differentiation while still creating an acceptable output color to optimize a color image reproduction.